Song is reportedly an employee at the Pyongyang University of Science and Technology.

RT.com has contacted the US Department of State for comment.

Song’s arrest follows the detention of US national Kim Sang Dok last month on charges of “criminal acts of hostility”. Dok, who is in his 50s, was reportedly arrested at Pyongyang International Airport while trying to leave North Korea.

The professor had been in North Korea to teach at the same university as Kim Hak Song.

A total of four US citizens are now thought to be in custody in North Korea.

The latest arrest comes at a time of heightened tensions between the US and North Korea, with each flexing their military muscles in recent weeks.

US Secretary of State Rex Tillerson said in March that his country’s “strategic patience” with North Korea has ended, and suggested military action was one of “options on the table.”

Stricter sanctions against North Korea, which could see US Navy vessels intervene with trade to and from the nation, have also been approved by the US House of Representatives.